An aerobic biological treatment of aqueous wastes by biodegradation of the organic substances in the waste using aerobic microorganisms, such as the so-called activated sludge treatment, permits a low cost operation with a superior treatment performance and has found a wide use. It suffers, however, from a problem of the occurrence of a large amount of “excess sludge” formed in accompaniment with such an aerobic biological treatment, which is difficult to dewater up to a degree permitting easy disposal or further processing thereof. The excess sludge formed during the biological treatment may amount to about 30-60% by weight of the biolyzed BOD in the treated waste, so that the disposal thereof brings about a practical problem. Heretofore, such an excess sludge has been disposed by depositing it in a landfill. In practice, it becomes more and more difficult to reserve such a landfill site, so that it is required to develop a technique for reducing the amount of the excess sludge to be formed.
To achieve such a reduction of excess sludge formation, it has generally been practiced to employ a biological digestion of the excess sludge formed in a conventional aerobic biological treatment of aqueous wastes, either under an aerobic or an anaerobic condition. This is accomplished by installing a biosludge digestion unit separately from the aerobic biological treatment apparatus for aqueous organic waste, in which the digestion of the biosludge is effected under an aerobic or anaerobic condition.
Such a practice provides a decomposition of the biosludge only up to a proportion of about 50% thereof, with the remainder of about 50% being discharged out of the system as a so-called “digested sludge” which is impervious to biological attack and should be disposed by incineration or burial.
In Japanese Patent Application Kokai Nos. 8835/1980, 105897/1984, 112899/1984 and 222798/1990, a technique for treating an excess sludge by first subjecting it to an oxidative decomposition by ozone and, then, biologically digesting the remaining sludge under an aerobic or anaerobic condition is disclosed and it is taught that the performance of reduction of the excess sludge amount is increased or the requisite digestion time can be decreased by the ozone treatment.
In these conventional techniques, however, an ozone treatment is employed for transforming the biosludge into BOD, so that a problem still remains in that a large amount of ozone is required for sufficient transformation of the biosludge into BOD.
Japanese Patent Application Kokai No. 206088/1994 discloses a technique for an aerobic biological treatment of aqueous organic wastes in which the biosludge is subjected to an aerobic biodegradation after it has been pretreated by an oxidative decomposition with ozone, whereby the reduction in the excess sludge amount is increased and, in some cases, even a complete elimination of the occurrence of excess sludge can be achieved. However, this Kokai does not disclose an ozone treatment carried out at an acidic pH condition of 5 or below.